The overall goal of this project is to gain a more complete understanding of Ca2+ homeostasis and signaling processes in arterial smooth muscle cells and to study the mechanism underlying hypoxic induced pulmonary vasoconstriction. The high-resistance arteries of the pulmonary arteries constrict shunting blood to well-oxygenated areas of the lung while renal arteries relax increasing flow, as do most vascular beds. By studying smooth muscle cells from pulmonary and renal arteries, mechanisms specific to the pulmonary vasculature can be discriminated from those that are more general. Specifically, three hypotheses will be tested: 1) That Ca2+ homeostasis in pulmonary arterial smooth muscle cells (PASMC) is governed by interactions between sarcoplasmic reticulum (SR), mitochondria, and extracellular Ca2+ sources; 2) That Sr Ca2+ release alters the membrane potential and that hypoxia modulates this process in PASMC; and 3) That intracellular pH changes affect Ca+ signaling and that hypoxia alters intracellular pH in PASMC. These hypotheses will be tested in freshly isolated canine pulmonary and renal arterial smooth muscle cells using whole-cell and laser-confocal fluorescence microscopy in combination with cell electrophysiology techniques. This study will help fill an existing gap in knowledge regarding the role of Ca2+ signaling processes in vascular reactivity of the pulmonary circulation. The medical significance is that the results obtained may reveal important cellular mechanisms responsible for pulmonary and systemic hypertension and could lead to the development of new drugs and therapeutic strategies to treat or prevent these conditions.